In an automatic transmission (AT) and a continuously variable transmission (CVT) for automotive vehicle, in order to hold oil pressure, there is arranged a seal ring which serves to seal an annular gap between a shaft and a housing which rotate relative to each other. A seal ring according to a conventional example will be explained while referring to FIG. 138 and FIG. 139. FIG. 138 is a schematic cross sectional view showing a state in which oil pressure is not held in the seal ring according to the conventional example. FIG. 139 is a schematic cross sectional view showing a state in which oil pressure is held in the seal ring according to the conventional example. A seal ring 6000 according to the conventional example is fitted in an annular groove 4100 formed in an outer periphery of a shaft 4000. And, the seal ring 6000 seals an annular gap between a shaft hole in a housing 5000 and the shaft 4000 by making sliding contact with an inner peripheral surface of the shaft hole in the housing 5000 through which the shaft 4000 is inserted, and with a side wall surface of the annular groove 4100, respectively.
The seal ring 6000 used for the above-mentioned usage is required to make sliding torque sufficiently low. For that reason, the peripheral length of an outer peripheral surface of the seal ring 6000 is made shorter than the peripheral length of the inner peripheral surface of the shaft hole in the housing 5000. In this manner, the seal ring 6000 is constructed such that it does not have a tightening margin with respect to the inner peripheral surface of the shaft hole. Then, in a state where an engine of an automotive vehicle is started to operate and the oil pressure is high, the seal ring 6000 is expanded in its diameter due to the oil pressure, so that it makes intimate contact with the inner peripheral surface of the shaft hole and the side wall surface of the annular groove 4100. As a result of this, a function to hold the oil pressure is exhibited to a sufficient extent (refer to FIG. 139).
In contrast to this, in a state where the oil pressure is not applied due to stoppage of the engine, the seal ring 6000 is in a state apart or separated from the inner peripheral surface of the shaft hole or side wall surfaces of the annular groove 4100 (refer to FIG. 138). For that reason, in the state where the oil pressure is not applied, the seal ring 6000 does not exhibit a sealing function. Accordingly, in a construction in which transmission control is carried out by means of oil pressure fed by a hydraulic pump, as in the case of an AT or a CVT, the oil having been sealed by the seal ring 6000 will return to an oil pan in an unloaded condition in which the hydraulic pump is stopped (for example, at the time of idling stop). As a result of this, the oil in the vicinity of the seal ring 6000 will be lost. Thus, when the engine is started (restarted) from this state, there will be no oil in the vicinity of the seal ring 6000, and engine operation will be started in a state where there is no lubrication. For that reason, there is a problem that responsiveness and operability are bad.